Growth hormone (GH) and cortisol responses to perturbation of the hypothalamic-pituitary-somatomedin axis have been widely used in psychiatric research as a peripheral correlate of central noradrenergic activity. 'Blunted' GH-responses are generally attributed to post-synaptic noradrenergic receptor down-regulation. We have administered clonidine to 31 healthy controls, 16 patients meeting DSM-III-R criteria for social phobia and 13 patients meeting DSM-III-R criteria for panic disorder. In addition, growth hormone-releasing factor (GRF) was administered to a subset of 21 normal controls and the 13 panic disorder patients. Compared with normal controls, both patients with social phobia and panic disorder exhibited 'blunted' GH responses whether examining log-transformed GH data or proportion of patients with + GH responses (defined as a change in GH >5 ng/ml). In addition, the panic disorder patients had blunted GH-response to GRF, replicating work by Rapaport et al.